System and method for controlling air flow through a powder coating booth

ABSTRACT

According to the invention, a system and method for controlling air flow through the interior of a powder spray booth includes a powder collection system for collecting oversprayed powder from the interior of the spray booth. The powder collection system includes a powder collector with a powder collection chamber, a pulse plenum chamber, and a fan plenum chamber containing a motor driven fan for drawing the air-entrained powder into the fan plenum chamber so that the oversprayed powder is collected on the cartridge filters and filtered air is exhausted from the fan plenum chamber through one or more final filters. A control system automatically adjusts the speed of the motor driven fan in response to pressure changes in the pulse plenum chamber and the fan exhaust chamber so that a substantially constant air flow is maintained through the powder collector.

FIELD OF THE INVENTION

The invention relates to make-up type spray booths, and moreparticularly, to a spray booth for applying powder coating material ontoarticles and a powder collector for collecting oversprayed powder fromsaid spray booth which are operated in conjunction with each other sothat a constant air flow is maintained through the powder spray booth.

BACKGROUND OF THE INVENTION

Powder spray systems are well known in the art and representativesystems are disclosed, for example in U.S. Pat. No. 5,261,934 ('934),assigned to Nordson Corp. of Westlake, Ohio, the assignee of thisinvention, which patent is incorporated in its entirety herein. Asdisclosed in the patent '934, the practice of powder coating involvesspraying a powdered coating material onto an object or workpiece andthereafter heating the object and the powder so that the powder melts.When subsequently cooled, the melted powder forms a solid, continuouscoating on the object. In many instances, an electrostatic charge isapplied to the sprayed powder and the object is electrically grounded toincrease the quantity of powder which attaches to the object and toassist in retaining the powder thereon.

Powder deposition is usually performed in a spray booth, i.e., anenclosure wherein any oversprayed powder which is not deposited on theobject can be collected. Conventionally, the containment of oversprayedpowder in the spray booth is aided by an exhaust system which creates anegative pressure within the spray booth and draws the powder entrainedin a stream of air out of the spray booth into a powder recovery unit,also called a powder collector. In the powder collector, the particlesof powder are separated from the air by a filter media, collected in ahopper, and then usually returned to the powder supply for sieving andrecirculating to the spray gun. The resulting cleaned air, now free ofthe powder, is usually passed through final filters and discharged intothe room or recirculated back to the conditioned air supply for thespray booth.

One problem associated with make-up spray booths of the type describedabove is to obtain a constant velocity air flow regardless of thevariation in the available volume for air flow through the interior ofthe booth, while concurrently ensuring that contaminants do not enter orleave the booth interior. This type of constant velocity air flow withinthe spray booth is desirable to minimize disruption of the flow pathbetween the powder dispensing devices and the object so that a uniformcoating is obtained on the object and to further ensure that a highpercentage of the amount of powder being sprayed attaches to the object,i.e., a high transfer efficiency.

It has been suggested that one way of obtaining such constant velocityflow conditions within the spray booth is to operate an air infeedblower and an air exhaust fan at a relative speed with respect to eachother so that the quantity of air entering the booth is equal to thequantity of air withdrawn from the booth. A problem with this design isthat no provision is made for accommodating changing conditions withinthe booth interior caused by the number and position of the objectsmoving therethrough. That is, no accommodation is made for variations inthe volume of air required to maintain constant downdraft velocity inthe spray booth as the progression of a object through the applicationarea displaces actual make-up air volume requirements.

These limitations have been addressed to some extent in systems of thetype disclosed in U.S. Pat. No. 4,653,387 to Osawa et al. This patentdiscloses air feed-type paint spray booths in which the air flow throughthe booth is varied in accordance with sensed conditions in the boothinterior such as booth pressure and/or the air velocity at the inlet andoutlet of the booth. For example, the Osawa patent includes an airinfeed fan and an air exhaust fan whose speed of operation are varied inresponse to the air flow sensed at the inlet and/or outlet to the booth.One problem with this design is the inaccuracy of the pressure and/orvelocity measurements taken within the interior of the spray booth,particularly when coating physically large objects which displacesubstantial quantities of air in the course of movement through thebooth, i.e., as the object enters or exits the booth compared tointervals when no object is present at the booth inlet or outlet.Another problem is that the air velocity can be so low in the boothinterior that it is difficult to measure, which in turn can result inthe velocity of the air moving through the booth being incorrectlyadjusted because the speed of the air infeed blower and/or air exhaustfan is dependent upon such measurements of velocity.

Another type of make-up type spray booth is disclosed in U.S. Pat. No.5,095,811 ('811) to Shutic et al., assigned to Nordson Corporation, theassignee of the present invention, which patent is incorporated in itsentirety herein. In the design of the '811 patent, the interior of thespray booth is divided into separate coating zones divided by transitionzones. The air infeed and exhaust devices associated with each coatingzone and each transition zone are operated to vary flow rate within thebooth interior in the course of movement of the item passing throughsuch that the air velocity in each coating zone is maintained below apredetermined maximum downdraft velocity throughout the coatingoperation, and such that a slightly negative pressure is maintainedwithin the booth interior.

While the previously discussed booth designs have partially addressedthe problem of providing a constant air flow through a spray booth,there are still deficiencies in these booth designs relating to thecontrol of the air flow through the powder coating booth to offset thechange in air flow corresponding to the build-up of coating powder onthe exterior surface of the cartridge filters in the powder collectorfrom the passage of the air entrained stream of oversprayed powder beingdrawn from the booth into the powder collector. That is, the amount ofair which can be drawn through the filters is directly dependent uponthe build up of powder on the cartridge filter. This variation in airflow causes problems both at start up when the filters are clean and ahigh volume of air can be drawn the filters and then after some periodof time when the powder buildup allows a much lower volume of air to bedrawn therethrough. Also, the filters are periodically subjected topulse cleaning when a blast of air, directed through the inside of thefilter, causes the accumulated powder on the outside surface of thefilter to fall off. This, will cause a rapid rise in the volume of airflow through the filter and often change the air flow within the powderbooth. As previously mentioned, changes in the air flow through thebooth can reduce the transfer efficiency and change the spray patternfrom the spray guns.

This problem is partially resolved by seasoning the cartridge filters,that is spraying the cartridge filters with powder prior to coatingparts with powder, so that the air flow is initially reduced. Cartridgefilters can require from 3 to 30 pounds of powder during seasoning.While the variation in the air flow through the spray booth is reduced,seasoning is a time consuming, expensive operation. The reduction invariations in air flow within the powder coating booth also lessens theeffect on the spray patterns emitted from the spray gun. Further, it isdesirable to improve the transfer efficiency of the powder coatingmaterial onto the parts being sprayed by closely controlling the speedof the fan drawing the oversprayed powder into the powder collector fromthe powder booth.

Another problem relating to the prior art booth designs is that themotor operated fans, used to draw the stream of air entrained,oversprayed powder from the booth into the powder collector, use a greatdeal of power and generate a high level of noise.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method and systemfor maintaining a substantially constant air flow through the interiorof a powder coating booth by regulating the fan speed in a powdercollector which obviate the problems and limitations of the prior artsystems.

It is a further object of the present invention to provide an improvedmethod and system for maintaining a substantially constant air flowthrough a powder collector and the interior of a powder coating booth toincrease the transfer efficiency of the coating powder onto the objectsbeing sprayed in the coating booth.

Yet another object is to provide method and system for maintaining asubstantially constant air flow through the interior of a powder coatingbooth to eliminate the need for seasoning the cartridge filters in apowder collection system prior to system startup.

Still another object is to provide method and system for maintaining asubstantially constant air flow through the interior of a powdercollector so that the operating cost of the system is reduced.

Another object is to provide method and system for maintaining asubstantially constant air flow through a powder collector by pullingair entrained powder from a powder coating booth so that the sound levelof the system is reduced.

According to the invention, a system for controlling air flow throughthe interior of a powder spray booth includes a powder collection systemlocated adjacent to the powder spray booth for collecting oversprayedpowder from the interior of the spray booth. The powder collectionsystem includes a powder collector having a powder collection chamberwith an exhaust opening and one or more air inlet openings with one ormore cartridge filters mounted thereto. A pulse plenum chamber having anexhaust port is connected to the air inlet openings. Air pulse elementswithin the pulse plenum chamber direct pulses of air through the inletopenings and into the cartridges. A fan plenum chamber has a fan inletwith a fan inlet cone connected to the pulse plenum chamber and a fanexhaust outlet. A motor driven fan within the fan plenum chamber drawsair-entrained powder into the collection chamber so that oversprayedpowder is collected on the cartridge filters and filtered air is drawnthrough the pulse plenum chamber, through the fan inlet cone into thefan exhaust chamber, and out of the fan exhaust outlet through one ormore final filters. A control system automatically adjusts the speed themotor driven fan in response to pressure changes in the pulse plenumchamber and the fan exhaust chamber so that a substantially constant airflow is maintained through the powder collector.

According to the invention, a first embodiment of the control system toadjust the speed of the fan includes a differential pressure transducerto generate an inlet cone velocity pressure corresponding to thedifference between the static pressure signal measured in the throat ofthe fan inlet cone and the total pressure signal corresponding to thepressure in the pulse plenum chamber. The difference between thesesignals is the inlet cone velocity pressure. The control system includesa controller receiving a signal corresponding to the inlet cone velocitypressure from the differential pressure transducer for calculating theactual air flow through the fan inlet cone with an equation where theactual airflow is proportional to the square root of the velocitypressure. Then, the actual air flow is compared to a preset commandedair flow to provide a resulting airflow differential. The resultingairflow differential is added to the preset commanded airflow togenerate an adjusted air flow command. A fan speed signal correspondingto the adjusted air flow command air flow through the spray booth isoutputted. A drive device, such as a frequency drive, receives the fanspeed signal and adjusts the speed of the motor driven fan. Also, thecontrol system can manually set the speed of the motor driven fan.

According to a second embodiment of the invention, the control system toadjust the speed of the fan includes two differential pressuretransducers to output a pulse plenum pressure signal corresponding tothe pressure in the pulse plenum chamber and a fan plenum pressuresignal corresponding to the pressure in the fan plenum chamber. Thepulse plenum pressure signal and the fan plenum pressure signal aretotaled together in a controller to determine a total pressure. Thecontroller then a) determines the actual air flow through the powdercollector, b) compares the actual air flow through the powder collectorto a desired air flow through the powder collector, and c) generates afan speed signal corresponding to the desired air flow through the spraybooth. Then, as in the first embodiment, a drive device receiving thefan speed signal adjusts the speed of the motor driven fan.

According to the invention, the method of controlling air flow throughthe interior of a powder spray booth, comprises the following steps. Theoversprayed powder is collected from the interior of the spray booth ina powder collection system located adjacent to the powder spray booth.Air-entrained, oversprayed powder drawn through an exhaust opening in apowder collector is collected from the spray booth on one or morecartridge filters mounted to air inlet openings in the powder collector.Next, clean air filtered through the cartridge filters is drawn into apulse plenum chamber. The clean air is then drawn through a fan inlet ofa fan plenum chamber with a motor driven fan connected by a fan inletcone to the fan inlet. The clean air is exhausted through a fan exhaustoutlet from the pulse plenum chamber through one or more final filters.The speed of the motor driven fan is automatically adjusted in responseto pressure changes in the pulse plenum chamber and the fan exhaustchamber so that a substantially constant air flow is maintained throughthe powder collector.

According to one embodiment of the invention, the step of adjusting thespeed of the motor driven fan includes the following steps. A velocitypressure signal is generated corresponding to the difference between thestatic pressure in the fan inlet cone and the total pressure in thepulse plenum chamber. Using the velocity pressure signal, the actual airflow through the fan inlet cone is next determined. The actual air flowis compared to a command air flow and an adjusted air flow command isdetermined. A fan speed signal is next generated corresponding to theadjusted air flow command through the powder collector. Then, the speedof the motor driven fan is adjusted.

In a second embodiment, the step of adjusting the speed of the motordriven fan includes the following steps. A total pressure signal equalto the combined pulse plenum pressure and the fan plenum pressure isdetermined. The total pressure is compared to fan curve equations andthe actual air flow through the powder collector is determined. Theactual air flow is compared to a preset commanded air flow through saidpowder collector and a fan speed signal is generated corresponding tothe desired air flow through the powder collector. Then, the speed ofthe motor driven fan is adjusted so that a substantially constantairflow substantially is maintained through said powder collectionsystem and said spray booth.

BRIEF DESCRIPTION OF THE DRAWINGS

The structure, operation, and advantages of the presently preferredembodiment of the invention will become further apparent uponconsideration of the following description taken in conjunction with theaccompanying drawings, wherein:

FIG. 1 is a schematic illustration of a powder collection systemincluding a powder coating booth and a powder collector which includes asystem for controlling air flow through the interior of the coatingbooth, in accordance with the invention;

FIG. 2 is a schematic illustration showing a portion of the powdercollector of FIG. 1 with an enlarged control system, in accordance withthe first embodiment of the invention;

FIG. 3 is a typical fan curve for the control system shown in FIG. 2showing the relationship between the pressure and airflow depending onthe accumulation of powder on the filter cartridges;

FIG. 4, comprised collectively of FIGS. 4A and 4B, is a flow chartillustrating the operation of the control system shown in FIG. 2;

FIG. 5 is a schematic illustration showing a portion of the powdercollector of FIG. 1, with an enlarged control system, in accordance withthe second embodiment of the invention;

FIG. 6 shows typical airflow curves for the control system shown in FIG.5 showing the relationship between the pressure and the fan speeddepending on he accumulation of powder on the filter cartridges; and

FIG. 7, comprised collectively of FIGS. 7A and 7B, is a flow chartillustrating the operation of the control system shown in FIG. 5.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, a powder coating system 10 is illustrated. Theoverall construction of the powder coating system 10 forms no part ofthis invention per se and is described only briefly herein. A morethorough description of the overall system is described in the U.S. Pat.No. 5,261,934 patent.

The powder coating system 10 includes a spray booth 12, a powdercollector unit 14, a powder supply 16, powder spray guns 18A,18B,18C,and a control system 22. The spray booth 12 is illustrated schematicallyand typically comprises an enclosed spray chamber 24 having side walls26, 28, a ceiling 30 and a floor 32. One or more gun openings 34 areprovided in the side wall 26. An exhaust opening 36 in the side wall 28provides a passageway through which air-entrained, oversprayed powder isdrawn into the powder collector 14, as discussed below. One or morespray guns 18A,18B,18C, connected to powder supply 16 and a power supply38, project through gun opening 34 in side wall 26 and are aimed at theinterior of spray chamber 24. A conveyor 40, supporting a workpiece 42on a conventional fixture 45, moves the workpiece to be sprayed pastspray guns 18A,18B,18C.

Powder collector 14 includes a powder collection chamber 44 having anexhaust opening 46, a plurality of air inlet openings 48, and afluidizing air plenum 50, disposed below the chamber 44, and separatedfrom chamber 44 by a fluidizing plate 51. A pump (not shown), pumps airinto the air plenum 50 and through fluidizing plate 51 so thatoversprayed particles collected within chamber 44 are fluidized. Acollection trough 53 is located in the lowermost portion of the chamber44, directly above fluidizing plate 51, to collect the powder. A pump 52(not shown) is connected to collection trough 53 through outlet 55 forpumping the collected powder back to feed hopper 57 of powder supply 16through line 54 for sieving and recirculation through line 56 to sprayguns 18A,18B,18C.

A plurality of stacked primary cartridge filters 58 are mounted to airinlet openings 48 at the top wall of collection chamber 44, by meanssuch as discussed in the '934 patent, for collecting air-entrained,oversprayed powder drawn through exhaust opening 46 from the enclosedchamber 24 of spray booth 12. While only two stacks of cartridge filtersare illustrated, it is within the terms of the invention to provide anynumber of cartridge filters, stacked or not stacked, as required.

With reference to FIG. 1, powder collector 14 includes a pulse plenumchamber 60 having an exhaust port 62 and inlet ports 64 being connectedto air inlet openings 48. Two air pulse elements 66A and 66B, connectedto a source of pressurized air (not shown), are mounted to extend withinchamber 60. Each air pulse element 66A, 66B includes a nozzle 68A,68Baligned with the inlet openings 48 through the lower wall 70 so thatpulses of air can be sequentially directed into cartridge filters 58 toknock off accumulated powder from the outer surface of the filters andonto collection trough 53 located above the fluidizing plate 51 inchamber 44 to collect the powder, as discussed before. Each air pulseelement 66A,66B is connected by a signal line 72A,72B to controller 22which sequences the opening and closing of the pulse elements. While asingle pulse plenum chamber 60 is illustrated, it is within the scope ofthe invention to provide a plurality of pulse plenum chambers, as shownand described in the '934 patent.

The fan exhaust chamber 90 of powder collector 14 has a fan inletconnected to the exhaust port 76 of pulse plenum chamber 60 and a fanexhaust outlet 94. Normally, a plurality of final filters 96 areprovided at the outlet 94 of exhaust chamber 90 for filtering any fineparticles of powder which were not removed by the filter cartridges 58.Fan exhaust chamber 90 has a motor driven fan 98, such as a SAFK FAN,manufactured by Chicago Blower Corporation, mounted to an outlet sectionof inlet cone 100, which in turn is secure at an inlet section to faninlet. Fan 98 is driven by a motor 102 through a conventional belt andpulley arrangement 104 to draw the air-entrained powder from powderbooth 12 into powder collection chamber 44 so that the oversprayedpowder is collected on cartridge filters 58 and filtered air is drawnthrough the pulse plenum chamber 60, into fan exhaust chamber 90, andout of fan exhaust outlet 94 through final filters 96.

Two different embodiments of control 22 are described below to controlthe airflow through powder booth 12, as shown in FIG. 1. In bothembodiments, the air flows through booth 12 and the oversprayed airentrained powder is drawn through into powder collector 14 to beseparated by filters 58 into powder for recycling back to feedhopper 57and cleaned filtered air to be exhausted through filters 96 and into theroom or recirculated back to the conditioned air supply for the spraybooth.

To better understand the operation and advantages of the presentinvention, the theory of operation for control system 22 is setforth.Referring to FIG. 1, airflow generated by fan 98 is used to contain thepowder in spray booth 12 and to pull the air-entrained oversprayedpowder into the powder collector 14. When fan 98 runs at a constantspeed, the airflow through spray booth 12 and powder collector 14decreases as powder collects on filter cartridges 58 to cause the filterpressure to increase, as shown in FIG. 3. The initial airflow in cubicfeet per minute (CFM) is higher when the cartridges are unseasoned.After seasoning, system 10 reaches an operating airflow range. However,as spraying continues, cartridges 58 typically get overloaded withpowder which, in turn, lowers the airflow through spray booth 12. Thebooth opening face velocity (the velocity across an open area of thebooth), which is proportional to the airflow, establishes the sprayconditions. If the airflow, or face velocity, is too high, the powderspray pattern can be pulled away from the part being sprayed causing thepowder transfer efficiency to be reduced. Conversely, when the airflowor face velocity is too low, all of the powder will not be contained inthe booth and some will leak out through the inlet or outlet openings.Therefore, the airflow balance can be critical in a powder sprayoperation.

As illustrated in FIG. 3, the initial airflow in cubic feet per minute(CFM) is higher when filter cartridges 58 are unseasoned. Afterseasoning (loading the cartridges with powder coating material), thesystem 10 reaches an operating airflow range. However, as sprayingcontinues, the filter cartridges typically get overloaded with powderand cause a reduction in the airflow through spray booth 12. The boothopening face velocity, which is proportional to the airflow, establishesthe spray conditions. If the airflow, or face velocity, is too high, thepowder spray pattern can be pulled away from the part causing areduction in powder transfer efficiency. Conversely, if the airflow orface velocity is too low, the powder being sprayed from guns 18A, 18B,18C will not be contained in the booth but will tend to escape from theentrance and exit openings through which the part enters and exits thebooth. Therefore, the airflow balance is an extremely importantconsideration in a powder spray operation.

The face velocity is also dependent on the booth opening area. Fan 98 issized to contain powder for a range of booth opening areas. Therefore,variations between systems will cause changes in the face velocity. Evenwhen the face velocity is sufficient to contain powder for booths of allsizes, some powder might be pulled away from guns 18A-18B beforereaching the part 42.

The control system 22, as described below, solves the airflow balanceproblem by controlling the airflow in powder booth 12 by varying thespeed of fan 98 to account for variations in the size of spray booth 12and the degree to which cartridges 58 are loaded with powder. The properairflow, that contains powder within booth 12 and maximizes transferefficiency, is set by control system 22. First, the airflow is set toaccount for the variations in the booth openings, i.e. the size andshape of the entrance and exit openings. Second, as the powder collectson cartridges 58 causing an increase in pressure and a decrease in airflow, as shown in FIG. 3, the control system 22 increases the speed offan 98 to maintain a substantially constant airflow in powder booth 12.Also, whenever cartridges 58 are pulsed cleaned with air pulse elements66A,66B, control system 22 reacts by decreasing the speed of fan 98 tomaintain a substantially constant airflow.

Referring now to FIG. 2, there is illustrated a first embodiment of acontrol system 22 which includes a controller 110 such as a programmablelogic controller (PLC) or a microcomputer, a differential pressuretransducer 112, and a frequency drive 114. The controller 110 typicallyincludes an interface circuit (not shown), a mathematical processor (notshown), and a memory unit (not shown). The interface circuit hasanalog/digital converters at the input thereof for connection to thepressure sensors 118 and 120, and the air pulse elements 66A, 66B. Theinterface circuit can also have a digital/analog converter at the outputthereof for connection to frequency drive 114.

The mathematical processor within control device 22 is adapted toperform pre-determined mathematical operations upon receiving data fromthe pressure transducer 112 for delivering a control signal to afrequency control 114 to vary the speed of rotation of fan 98 throughcontrol of motor 102. The memory unit stores a pre-determined programfor performing the mathematical operation in the mathematical processor,together with various data required for such operation.

The differential pressure transducer 112 is connected by a line 116 to afirst pressure tap 118 mounted in the throat of inlet cone 100 formeasuring the cone throat static pressure P_(S) and by a line 117 to asecond pressure tap 120 in pulse plenum chamber 60 for measuring thetotal pressure measured P_(T) in the pulse plenum chamber. The actualairflow (Q_(A)) through the inlet cone 100 can be calculated with theequation Q_(A1),2,etc. =K×4005 √ΔP_(V), where airflow is proportional tothe square root of the inlet cone velocity pressure P_(V). The velocitypressure P_(V), is determined by subtracting the inlet cone throatstatic pressure P_(S) from the total pressure P_(T) measured in thepulse plenum chamber 60. P_(V) can be readily measured by differentialpressure transducer 112 which in turn generates a velocity pressuresignal. The velocity pressure signal is sent from transducer 112 tocontroller 110, where the velocity pressure signal is converted to thevelocity pressure. Then, the velocity pressure is mathematicallyoperated on by the equation Q_(A) =K×4005 √ΔP_(V) with the software incontroller 110 to calculate the actual total airflow through inlet cone100. The actual total airflow Q_(A) is then subtracted from a commandairflow Q_(C) through inlet cone 100, preset by an operator. Based onthis comparison, a fan speed signal is output by controller 100 forcontinually adjusting the speed of fan 98 so that the actual airflowthrough inlet cone 100 equals the desired airflow through inlet cone100. The effect of maintaining a constant command airflow Q_(C) throughinlet cone 100 is that a substantially constant airflow is maintainedthrough powder booth 12. The speed of fan 98 is varied by outputting thespeed signal from controller 110 to frequency drive 114 which in turnadjusts the motor drive frequency, and therefore the motor speed, vialine 115 to set the speed of motor 102. Motor 102 then rotates fan 98 ata speed (N) through the belt and pulley drive 104.

Airflow control system 22 includes three modes of operation options tooperate fan 98: a) a manual fan speed adjustment mode, b) a cleanupmode, and c) an automatic airflow control mode. The manual fan speedadjustment mode enables an operator to set the fan to any desired speed.The manual cleanup mode enables an operator to set the fan to themaximum speed so that the powder which is being blown around in thespray booth, as the latter is being cleaned with compressed air sprayedfrom an air hose, is exhausted out of the spray booth to be reclaimed inthe powder collector 14. The cleanup mode maximizes powder containmentin the powder booth 12. The automatic mode enables an operator to setcontrol system 22 to automatically maintain the airflow through powderbooth 12 at a desired preset value. In the automatic mode, controlsystem 22 determines the airflow through powder booth 12, based onpressure measurements in pulse plenum chamber 60 and fan exhaust chamber90 of powder collector 14, and then regulates the speed of motor 102 andfan 98 to compensate for changes in the airflow through the powdercollector away from the desired preset value. In addition, the controlsystem 22 can include a pulse on demand mode which is essentially astand-alone section of controller 110 that includes a cartridgesequential pulsing circuit (not shown) that sequences the opening andclosing of the air pulse elements 66A,66B whenever a set pressure rangeis reached across said cartridge filters 58.

Referring to FIG. 4, there is illustrated a flow chart showing theprocessing steps performed in control system 22, shown in FIG. 2. Thefirst mode to be considered is the manual mode. Based on signals from aninput of the manual mode selection at step 1 and the setpoint selectionat step 2, the manual fan speed adjustment at step 6 is set and the fanspeed at step 7 is determined, respectively. This in turn generates asignal to use the manual fan speed at step 8. The resulting manual fanspeed signal which is output at step 8 is converted at step 9 to a motorspeed signal. The motor speed signal is next converted to a motorfrequency signal at step 10 and a signal corresponding to a frequencyequivalent of the desired motor speed is outputted to frequencycontroller 114. A resulting frequency signal is then sent through line115 to set the motor 102 to the desired speed so that fan 98 draws thedesired air flow through powder collector 14 and in turn through powderbooth 12.

The next mode to be considered is the cleanup mode. Based on an input ofthe cleanup mode selection at step 1, the cleanup mode at step 3 sends asignal to set the maximum fan speed at step 4. This in turn directs amaximum fan speed signal to be converted to a corresponding motor speedat step 9. The resulting motor speed signal which is output from step 9is converted to a corresponding motor frequency at step 10. Then, amotor frequency signal is outputted to frequency controller 114, whichin turn generates a frequency signal corresponding to a desired motorspeed. The motor frequency signal is sent through line 115 to set thespeed of motor 102 so that fan 98 rotates at the desired maximum speedto generate an air flow command Q_(c) that pulls the air entrainedpowder from spray booth 12 into powder collector 14.

The next significant mode of operation to be discussed is the automaticmode. In this mode, based on an input of the automatic mode at step 1and the setpoint selection at step 2, the automatic mode is selected atstep 11 and a signal corresponding to the desired airflow command(Q_(C)) through powder collector 14 is generated at step 12. The inletcone velocity pressure at the throat of inlet cone 100 is determined incontroller 110 at step 13. Then, the velocity pressure is mathematicallyoperated on by the equation Q_(A) =K×4005 √ΔP_(V) with the software incontroller 110 to calculate the actual total airflow Q_(A) through inletcone 100 at step 14. The actual total airflow Q_(A), as calculated atstep 14, is then subtracted from airflow command Q_(C) at step 15. Theresulting airflow differential is then added to the airflow commandQ_(C) at step 16 to generate an adjusted air flow command Q_(AC). Theadjusted airflow command is then scaled to a corresponding fan speed atstep 17. This fan speed signal is next compared to a maximum fan speedat step 18 and the lower of the two is selected. The resulting fan speedsignal is next compared with the minimum fan speed and the higher of thetwo is selected at step 19. The final command fan speed signal N_(fc),in the automatic mode, which in turn is generated at step 20, isconverted to a signal corresponding to the desired motor speed at step9. The motor speed signal, in turn, is converted to a correspondingmotor frequency at step 10. The resulting frequency signal is outputtedto frequency controller 114 through line 115 to set the speed of motor102 so that fan 98 operates at the desired speed. The fan speed iscontinually adjusted with controller 22 so that the actual airflowthrough powder collector 14 equals the commanded airflow whereby asubstantially constant airflow is maintained through powder booth 12.

The use of a frequency drive 114 to adjust the speed of motor 102, whichin turn controls the air flow through fan 98, allows fan 98 to rotate ata speed that minimizes the motor electrical energy used to power motor102 and reduces the rotation of fan 98 and therefore the sound level,while maintaining the desired airflow through the powder booth.

Referring to FIG. 5, there is illustrated a second embodiment of theinvention in which the control system 22' uses a total system pressuredrop and a fan curve relationship to control the airflow. Throughout thespecification primed numbers represent structure elements which aresubstantially identical to structure elements represented by the sameunprimed number. The actual airflow Q_(A) is a function of the totalsystem pressure drop across cartridges 58, final filter 96, and thespeed of fan 98, as shown in FIG. 5. Control system 22' includes acontroller 110', such as a programmable logic controller (PLC) or amicrocomputer, two differential pressure transducers 112' and 131, and afrequency drive 114'. While two pressure transducers 112' and 131 areillustrated, it is within the terms of the invention to substitute asingle pressure transducer adapted to provide the function of twopressure transducers.

In the second embodiment, one of the transducers 112' is connected by aline 117' to a first pressure tap 120' mounted in pulse plenum chamber60 for measuring the pulse plenum pressure P_(pp) corresponding to thepressure across cartridge filters 58 and outputting a correspondingpulse plenum pressure signal to controller 110'. Second pressure tap 130is located in fan plenum chamber 104 for measuring the fan plenumpressure P_(fp), corresponding to the pressure across final filters 96and outputting a corresponding fan plenum pressure signal to controller110'. Controller 110' converts the pulse plenum pressure signal to apulse plenum pressure and the fan plenum pressure signal to a fan plenumpressure. Then, total pressure P_(T), is determined by adding the fanplenum pressure P_(FP) to the pulse plenum pressure P_(PP) in controller110'. The resulting total pressure P_(T) is related to the commandedtotal airflow Q_(C1),Q_(C2),Q_(C3),Q_(C4), . . . Q_(CN) desired throughpowder collector 14 and set by an operator. Each of the airflow linesQ_(C1) ,Q_(C2),Q_(C3), . . . , as shown in FIG. 6., is nonlinear anddetermined experimentally. However, the airflow lines have been found tobe best linearized and characterized by two separate linear equationswhich have been found empirically to be separated at a knee pressureP_(K). While the knee pressures P_(K) for each airflow line (Q_(CN)) isunique, they all occur at the same fan speed. The following equation:

    N=mP.sub.T +C

where

N=the motor speed

P_(T) =the total pressure

m=the slope of the air flow line

C=a constant (Y intercept)

forms two separate equations with two different values for the slope mand intercept C of each airflow line. All of the data needed tocalculate the fan speed is stored in the memory unit of controller 22'which also stores a pre-determined program for performing themathematical operation in the mathematical processor.

In operation, the actual airflow Q_(A) through powder collector 14 iscompared to a desired or command airflow Q_(C) through the powdercollector, preset by an operator in controller 22'. A fan speed unmannedsignal is the output from controller 110' so that the speed of fan 98 iscontinually adjusted to keep the actual airflow through powder collector14 equal to the command airflow, i.e., a substantially constant airflowis maintained through both powder collector 14 and powder booth 12.Controller 110' controls the speed of fan 98 by controlling the speed ofmotor 98 with frequency drive 114.

Referring to FIG. 7, there is illustrated a flow chart showing theprocessing steps performed in control system 22', shown in FIG. 5. Thefirst mode to be considered is the manual mode. Based on an input of themanual mode selection at step 1 and the setpoint selection at step 2,the manual fan speed adjustment at step 6 is set and the fan speed atstep 7 is determined, respectively. This in turn outputs a fan speedsignal which activates a control to use the manual fan speed at step 8.The fan speed signal is then converted to a motor speed signal at step9. The motor speed signal is next converted to a motor frequency signalat step 10, and a signal corresponding to a frequency equivalent of thedesired motor speed is outputted to frequency controller 114. Aresulting frequency signal is then sent through line 115 to set themotor 102 to the desired speed so that fan 98 draws the desired air flowthrough powder collector 14 and in turn through powder booth 12.

The next mode to be considered is the cleanup mode. Based on an input ofthe cleanup mode selection at step 1, the cleanup mode at step 3 sends asignal to set the maximum fan speed at step 4. The maximum fan speedsignal, in turn, is converted to the desired motor speed signal at step9. The motor speed signal is then converted to a motor frequency signalat step 10, which in turn is outputted to frequency controller 114 toset the motor 102 through line 115 to the desired maximum speed so thatfan 98 operates at the desired maximum speed to generate an air flowthat pulls the air entrained powder from powder booth 12 into powdercollector 14.

In the automatic mode of operation, based on an input of the automaticmode at step 1 and the setpoint selection at step 2 by an operator, theautomatic mode is selected at step 11 and the airflow command (Q_(C)) isdetermined at step 12 and the fan curve equations are activated in step12A. The fan plenum pressure P_(FP) across final filter 96 is inputtedat step 13 and the pulse plenum pressure P_(PP). across cartridgefilters 58 is inputted at step 14. The total pressure P_(T), isdetermined by adding the fan plenum pressure P_(FP) to the pulse plenumpressure P_(PP). at step 15. The resulting total pressure P_(T) is thencompared with the knee pressure P_(K), as shown in FIG. 6, at step 16.If P_(T) is less than P_(K), then the equation 1 (N=mP_(T) +C) with thesoftware in controller 110' to calculate the fan speed is selected atstep 17. As previously explained, equation 1 uses one value of m and C,as read from a table of values in the software of controller 110'.Conversely, if P_(T) is greater than P_(K), then equation 2 (N=mP_(T)+C) is selected at step 18 with second values of m and C. Next, usingeither equation 1 or 2, as previously determined, the total pressure ismathematically operated on by the formula N=mP_(T) +C, using theappropriate values of m and c, to calculate the fan speed at step 19.The signal corresponding to the fan speed N_(f) is next compared to amaximum allowable fan speed at step 20 and the lower of the two fanspeeds is selected. Continuing, the resulting signal from step 20 iscompared with a minimum allowable fan speed and the higher of the twofan speeds is selected at step 21. The resulting final fan speed commandsignal N_(fc), in the automatic mode, is outputted at step 22 andconverted to the desired motor speed at step 9. The motor speed signalis then converted to a motor frequency signal at step 10, which in turnis outputted to frequency controller 114 to set the speed of motor 102through line 115 so that the speed of fan 98 is continually adjusted sothat the air flow through powder collector 14 equals the commanded airflow and a substantially constant air flow is maintained through powderbooth 12.

According to the invention, it is apparent that there has been providedin accordance with this invention a method and system for maintaining asubstantially constant air flow through the interior of a powder coatingbooth by regulating the fan in a powder collector which obviate theproblems and limitations of the prior art systems. The substantiallyconstant air flow through the interior of a powder coating boothincreases the transfer efficiency of the coating powder onto the objectsbeing sprayed, eliminates the need for seasoning the cartridge filtersin a collector system prior to system startup, and reduces the noise andoperating cost of the system.

While the invention has been described in combination with embodimentsthereof, it is evident that many alternatives, modifications, andvariations will be apparent to those skilled in the art in light of theforegoing teachings. Accordingly, the invention is intended to embraceall such alternatives, modifications and variations as fall within thespirit and scope of the appended claims.

We claim:
 1. A system adapted for controlling air flow through theinterior of a powder spray booth, comprising:a powder collection systemadapted to be located adjacent to said powder spray booth for collectingoversprayed powder from said interior of said spray booth, said powdercollection system including:a powder collector having a powdercollection chamber with an exhaust opening and one or more air inletopenings, said powder collection chamber having one or more cartridgefilters mounted to said air inlet openings; a pulse plenum chamberhaving an exhaust port and being connected to said air inlet openings,said pulse plenum chamber having one or more air pulse elements fordirecting a pulse of air through said air inlet openings and into saidcartridge filters through said air inlet openings; a fan plenum chamberhaving a fan inlet connected to said pulse plenum chamber and a fanexhaust outlet, said fan inlet having a fan inlet cone mounted theretoand a motor driven fan mounted within said fan plenum chamber fordrawing air entrained powder from said powder spray booth through saidexhaust opening into said powder collection chamber so that saidoversprayed powder is collected on said cartridge filters and filteredair is drawn through said pulse plenum chamber, into said fan plenumchamber and out of one or more final filters mounted to said fan exhaustoutlet; and a control system to automatically adjust the speed of saidmotor driven fan in response to pressure changes in said pulse plenumchamber and said fan plenum chamber of said powder collector so that asubstantially constant air flow is maintained through said powdercollection system.
 2. The system of claim 1 wherein said control systemto adjust the speed of said motor driven fan includes:a differentialpressure transducer to output a velocity pressure signal correspondingto the difference between the pressure in a throat of said fan inletcone and the pressure in said pulse plenum chamber; a controllerreceiving said velocity pressure signal from said differential pressuretransducer for determining the air flow through said fan inlet cone,subtracting said air flow through said fan inlet cone from a presetcommanded air flow to provide a resulting airflow differential andadding said resulting airflow differential to said preset commandedairflow to generate an adjusted air flow command, generating an adjustedair flow command based on said resulting airflow differential,outputting a fan speed signal corresponding to said adjusted air flowcommand; and a drive device receiving said fan speed signal foradjusting the speed of said motor driven fan so that a substantiallyconstant airflow is maintained through said powder collection system. 3.The system of claim 2 wherein said drive device is a frequency drive foradjusting the frequency of said motor driven fan and controlling thespeed of rotation of said motor driven fan.
 4. The system of claim 3further including first and second pressure taps in said pulse plenumchamber and in the throat of said inlet cone, respectively, connected tosaid pressure transducer.
 5. The system of claim 1 wherein said controlsystem can manually set the speed of said motor driven fan.
 6. Thesystem of claim 1 wherein said control system is operatively connectedto said one or more air pulse elements for sequencing their opening andclosing whenever a set pressure range is reached across said cartridgefilters.
 7. The system of claim 1 wherein said control system to adjustthe speed of said fan includes:a pressure transducer device to output apulse plenum pressure signal corresponding to the pulse plenum pressurein said pulse plenum chamber, said pressure transducer device outputtinga fan plenum pressure signal corresponding to the fan plenum pressure insaid fan plenum chamber; a controller receiving said pulse plenumpressure signal and said fan plenum pressure signal from said pressuretransducer device and converting said pulse plenum pressure signal andsaid fan plenum pressure signal to pulse plenum pressure and fan plenumpressure, respectively, said controller determining a total pressure byadding said pulse plenum pressure with said fan plenum pressure anddetermining the air flow through said powder collection system relativeto the total pressure, said controller comparing said air flow to apreset commanded air flow and generating a fan speed command signalcorresponding to the desired air flow through said powder collectionsystem; and a drive device receiving said fan speed command signal foradjusting the speed of said motor driven fan so that the airflow throughsaid powder collection system substantially equals said command airflow.8. The system of claim 7 wherein said drive device is a frequency drivefor adjusting the frequency of said motor driven fan for controlling thespeed of rotation of said motor driven fan.
 9. The system of claim 7further including first and second pressure taps in said pulse plenumchamber and said fan plenum chamber connected to said pressuretransducer device for sensing pressure corresponding to the pressureacross said cartridge filters and said final filters, respectively. 10.The system of claim 9 wherein said pressure transducer device comprisesfirst and second differential pressure transducers, and said firstpressure tap in said pulse plenum chamber is connected to said firstdifferential pressure transducer and said second pressure tap in saidfan plenum chamber is connected to said second differential pressuretransducer.
 11. The method adapted for controlling air flow through theinterior of a powder spray booth, comprising the steps of:drawingair-entrained oversprayed powder from said interior of said spray boothinto a powder collection system located adjacent to said powder spraybooth, said step of drawing including the steps of:drawing saidair-entrained, oversprayed powder from said spray booth through anexhaust opening in a powder collection chamber of a powder collectorhaving one or more cartridge filters mounted to air inlet openings insaid powder collection chamber; drawing filtered air through saidcartridge filters into a pulse plenum chamber while said powder collectson said cartridge filters; drawing said filtered air through said pulseplenum chamber and through a fan inlet cone mounted to a fan inlet of afan plenum chamber with a motor driven fan mounted within said fanplenum chamber; exhausting said filtered air through one or more finalfilters mounted in a fan exhaust outlet from said pulse plenum chamber;and automatically adjusting the speed of said motor driven fan inresponse to pressure changes in said pulse plenum chamber and said fanplenum chamber so that a substantially constant air flow is maintainedthrough said powder collection system.
 12. The method of claim 11wherein said step of adjusting the speed of said motor driven fanincludes the steps of:generating a velocity pressure signalcorresponding to the difference between the pressure in said fan inletcone and the pressure in said pulse plenum chamber; determining the airflow through said fan inlet cone; subtracting said air flow through saidfan inlet cone from a preset commanded air flow to provide a resultingairflow differential and adding said resulting airflow differential tosaid preset commanded airflow to generate an adjusted air flow command;outputting a fan speed signal corresponding to said adjusted air flowcommand through said powder collector; and adjusting the speed of saidmotor driven fan so that a substantially constant airflow is maintainedthrough said powder collection system.
 13. The method of claim 12wherein said step of determining the air flow through said fan inletcone includes mathematically operating on said velocity pressure P_(V)by the equation Q_(A) =K×4005√ΔP_(V).
 14. The method of claim 12 whereinsaid step of adjusting the speed includes the step of adjusting thefrequency of said motor driven fan for controlling the rotational speedof said fan.
 15. The method of claim 13 further including the step ofsensing the pressure corresponding to said pressure in said fan plenumchamber with a first pressure tap located in said inlet cone and sensingthe pressure corresponding to said pressure in said pulse plenum chamberwith a second pressure tap located in said pulse plenum chamber.
 16. Themethod of claim 11 further including the step of manually setting thespeed of said motor driven fan.
 17. The method of claim 11 wherein saidstep of adjusting the speed of said motor driven fan includes the stepsof:determining a total pressure P_(T) corresponding to the combinedpulse plenum pressure and fan plenum pressure; determining the air flowthrough said powder collection system; comparing said air flow throughsaid powder collection system to a preset commanded air flow throughsaid powder collection system; generating a fan speed signalcorresponding to said preset commanded air flow through said powdercollection system; and adjusting the speed of said motor driven fan sothat a substantially constant airflow is maintained through said powdercollection system.
 18. The method of claim 17 wherein said step ofdetermining said air flow through said powder collection system includesthe steps of:comparing said total pressure P_(T) with a knee pressureP_(K) on an airflow line corresponding to said preset commanded airflow; selecting a first equation N=mP_(T) +C where N is the speed ofsaid motor driven fan and m and C have a first set of values when P_(T)is greater than P_(K) or a second equation N=mP_(T) +C where m and Chave a second set of values when P_(T) is less than P_(K;) ; andmathematically operating on said total pressure P_(T) by said selectedfirst or second equation to generate said fan speed signal.
 19. Themethod of claim 17 further including the step of adjusting the frequencyof said motor driven fan for controlling the speed of said fan.
 20. Themethod of claim 17 further including the step of sensing the pressurecorresponding to the pulse plenum pressure with a first pressure taplocated in said pulse plenum and sensing the pressure corresponding tosaid fan plenum pressure in said fan plenum chamber with a secondpressure tap.